Chris Pearson said:

Just a thought here. Is there any provision which specifies voltage drop at HV. Surely, if HV is low, Lv must also be low?

No specific provision in BS 7671 ... and agreed, but transformers with tap-changers and AVC can be used to overcome that (or, in the LV system, autotransformers with tap-changers)

Think we are drifting away from the question. What is a private supply in terms of BS7671? Option  1 or Option 2?

Peter Hartley said:

Think we are drifting away from the question. What is a private supply in terms of BS7671? Option  1 or Option 2?

Anything other than a "standard" ESQCR public LV supply is the answer. Not drifting, that was answered earlier, and the discussion progressed beyond it.

I have searched the web and found the definition of a private supply according to BS 7671. A private supply is a supply that comes from a source of energy other than a public electricity supply system, This definition is also in agreement with the one given in [Wikipedia], which references BS 7671:2018 as its source. However, I could not find the exact location of this definition in the BS 7671

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

I think there is some misunderstanding about my answer. I did not say that both Option 1 and Option 2 are private supplies. I said that only Option 2 is a private supply. Option 1 is not a private supply, because it still relies on the public electricity supply system at the HV level. The private transformer does not alter the source of energy, only the voltage level. Therefore, Option 1 is a public supply with a private transformer, not a private supply. 

It looks confusing in the chat. I actually replied to gkenyon. I understand what you are saying and thats what I thought. I'm also aware of the Internet definitions you quoted. I always thought the same as you. However a colleague said the same as Chris Pearson above. If you own the transformer you are able to adjust the tappings on the transformer to achieve peak efficiency on your volt drops. So I tend to agree with them. Just wish BS7671 was clear on this.

When we don’t have a clear answer, we call it a grey area. It’s like the weather in Manchester, always cloudy and uncertain. 

Peter Hartley said:

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

Yes, they could both be considered a private supply, although as I have said the over-riding factors are not really the drop within the installation, but conformity with Regulations 525.1 and 525.201.

Peter Hartley said:

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

Yes, they could both be considered a private supply, although as I have said the over-riding factors are not really the drop within the installation, but conformity with Regulations 525.1 and 525.201.

I’d go with Graham’s final conclusion. My opinion has been changed 

BS 7671 covers circuits supplied at nominal voltages up to and including 1000V AC or 1500V DC. It also applies to Extra Low Voltage (ELV) ranges and Low Voltage (LV) ranges, across a variety of frequencies.

Thus oprion 1 is null and void!

Or more to the point, HV supply is not taken into account, thus transformer would be private supply 

Respectfully I disagree with your response. A private supply is defined as a supply that comes from a non-public source of energy, The HV supply is still a public source of energy, even if it is not considered. The transformer does not alter the source of energy, only the voltage level. Therefore, the transformer is not a private supply, but a public supply with a private transformer. 

The key thing about a private transformer - one where only you have the secondary side - usually the HV is shared with others (!) , is that you are free to set the taps (or call in the DNO to set the taps to your needs, depending on contract agreement and who has suitably qualified staff )

This means that the voltage drop allowance normally reserved for the street cables is yours to play with - which if you have a large campus you will probably need to.

Mike

PS A private supply is defined as a supply that comes from a non-public source of energy, The HV supply is still a public source of energy, even if it is not considered.

I think the authors of BS7671, and a number of designers of large sites do not agree with that interpretation.  :-)

fully agree with you. l

I think we need to distinguish between supply/ origin and source. 525.202 clearly states the volt drop measurement should origin of the installation (usually the supply terminals) and the socket outlet....

BS7671 only applies to installations 1000v AC / 1500 DC and below. Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Annex 4 - 6.4, Table 4Ab - Voltage drop (ii) Low Voltage installation supplied from private LV supply.

The HV supply or the source of energy is not considered for the purposes of Volt drop, but Origin of the LV supply is.

If those terminals are privately owned, it would be considered a private supply.

But the DNOs themselves were privatised after they took over from the old Electricity Boards - they may be regulated, provide a supply to the public, but their equipment isn't publicly owned.

   - Andy.

Much like the railways!

Peter Hartley said:

Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Agreed, although ownership is not the issue because the transformer could be leased, but still be private.

It seems reasonable to regard the LV terminals of a private transformer as the origin, but otherwise it is the service head.

The real point is whether the transformer supplies any other customer or not. So if it is just the one, it is possible to compensate for voltage drop within the installation. However if there is more than one customer, if the wick is turned up because of voltage drop in one installation, the voltage may be too high for another so there is less scope for adjustment.